Using chlorophyll fluorescence to evaluate light reactions of photosynthesis for hardwood advanced reproduction in varying understory sunlight environments

Background/Question/Methods

Hardwood forests in the Arkansas Ozarks have been important culturally, ecologically and environmentally for centuries. Historically these forests were fire dependent and dominated by oak (Quercus) species. In the past century, fire suppression and forest management have caused these woodlands to become dense, low sunlight environments. As a result, Quercus species are increasingly less abundant following disturbance in natural hardwood stands. Many applied ecological studies have explored methods and practices to maintain Quercus species in newly developing stands. This study attempted to link the mechanistic physiology of Quercus and non-Quercus reproduction to the applied ecological work. Chlorophyll fluorescence (CF) was used to evaluate the photosynthetic response of advanced reproduction to varying sunlight conditions following a partial harvest in an upland hardwood stand. CF samples were taken from three Quercus species and three Non-Quercus competitors. Stand level treatments were post-harvest residual basal area 11.6 m² h^-1 (BA11), BA11 plus midstory control (BA11+MR), and non-harvest control (NHC). Treatment analyses were conducted on leaf level irradiance, quantum yield of PSII (ΦPSII), and electron transfer rate (ETR). Species level analyses were performed on cardinal points of photosynthetic light curves.

Results/Conclusions

Average leaf level irradiances for all years by treatment were 423.4 (SE=70.0), 677.4 (SE = 101), and 137.1 (SE=35 ) µmol m^-2 s^-1 for BA11, BA11+MR, and NHC, respectively. Average ΦPSII values were 0.581 (SE=0.042), 0.495 (SE=0.051), and 0.691 (SE=0.018) for BA11, BA11+MR, and NHC, respectively. Mean ETR values were 46.8 (SE= 14.1), 77.4 (SE= 20.1), and  49.7 (SE= 4.3) µmol m^-2 s^-1 for BA11, BA11+MR, and NHC, respectively. A one-way analysis of variance determined significant differences to exist (P < 0.001 for PPFD, ΦPSII, and ETR). Means separation (SNK method) detected a significant difference among all treatments for PPFD, ΦPSII, and ETR.  Mid-season, ambient ETR_max of advanced reproduction adapted to two growing seasons of partial sun/partial shade ranged from 143.8 for Quercus rubra to 231.1 µmol m-2 s-1 for Ulmus alata. Year two ETR_max values were higher than mid-season, year one values for all species excluding Acer rubrum which was reduced in year 2. Year 3 light curve analyses provided insight into a season of drought stress conditions.